This invention relates to the art of reading arrays of values, such as the values of pixels in a solid state image sensor.
When reading values from arrays, such as an array of pixels in a solid state image sensor or other sensor, often the elements of a column of the array are multiplexed onto a single line, and these column lines may then be further multiplexed onto a single serial readout line, such as one which is connected to an analog-to-digital (A/D) converter. The multiplexing is often achieved by using switching transistors which are addressable to control access to the column and/or readout line.
Because the feature size of silicon devices has become so small, it is possible today to include within each sensor array element some active circuitry, e.g., an amplifier or buffer, and arrays with such included active circuitry are known in the art as active arrays. Using such active circuitry is advantageous, because the signal from the sensor can be conveyed to the output line with low noise and with far less attenuation than with prior so-called passive arrays, which did not include active circuitry in the array elements. Often, the circuitry in each array element is arranged as a source follower which employs a current source that is located outside of the array element and is shared by all array elements in a column.
We have recognized that source followers implemented in complementary metal oxide semiconductor (CMOS), which is the currently preferred technology for sensor arrays due to cost considerations, suffer from a voltage gain of significantly less than one as a result of the so-called xe2x80x9cbody effectxe2x80x9d, an effect which causes the threshold voltage of the MOS transistor to vary with its source voltage. As a result, the signal from the sensor element is attenuated. Because of the attenuation, additional amplifiers are required to compensate for the attenuation. Unfortunately, use of such additional amplifiers causes the
signal-to-noise ratio of the sensor output to be degraded. Also, there may be variations in the gain from pixel to pixel because of variations in the body effect and other transistor characteristics which contribute to the determination of the gain. Such variations manifest themselves as fixed pattern noise (FPN). Although each transistor could be placed within its own substrate diffusion well to eliminate body effect, doing so significantly reduces the density of sensor array that is achievable.
These problems with source followers employed in active arrays may be alleviated, in accordance with the principles of the invention, by employing for each element a differential amplifier. However, instead of including an entire differential amplifier within each element, part of the differential amplifier structure is shared among each sensor element connected to a particular column. For example, in one embodiment of the invention, the differential amplifier may be a differential pair operational transconductance amplifier (OTA) which is connected in a feedback configuration. However, instead of including an OTA within each sensor element of the array, part of the OTA structure is shared among each sensor element connected to a particular column. Thus, each sensor element may include the same components as in the prior art when a source follower was employed, with the remaining elements necessary to complete the OTA being shared among all the sensor elements of the column. Although gains greater than one are achievable by employing the differential amplifier with a feedback network, to avoid introducing fixed pattern noise, substantially unity gain, i.e., a gain of approximately one, but typically slightly less than one, is often preferred.
We have also recognized, in accordance with an aspect of the invention, that the shared differential amplifier model of array reading may be employed for any active array readout application, e.g., including arrays that are not in themselves per se sensor arrays, where the necessary structure to form the shared differential amplifier is included within the elements of the array.